The Split Ring Resonator (SRR) is one of the most common unit cell configurations found in metamaterials. One of the main problems being faced with traditional SRR geometries is a lack of bandwidth. Additionally, an area of great interest would be attaining in some level of reconfigurability with a metamaterial surface.
Referring to FIG. 1a and its circuit equivalent in FIG. 1b, the traditional SRR configuration is a planar ring 10 with some number of gaps 20 in it. The E-field is most focused in these gaps. Typically, frequency tuning is achieved through either the addition of stubs, loading some lumped elements in the gap, or loading the outermost ring 10 with a number of concentric inner rings 30, 40, 50. Each of these has distinct advantages and drawbacks.
The addition of a planar stub (not depicted) can effectively create a planar-profile load on the split ring resonator, but this type of load is static and cannot be tuned. Typically, planar stubs are done on the exterior of the cell and cause an increase in the overall cell area. The alternative of adding lumped elements (not depicted) into the gap may provide more variation and ability to tune the SRR, especially with the use of varactors. These, however, can only be tuned to maximize response at one frequency at a given time. Finally, the addition of the interior concentric rings 30, 40, 50 provides a static load, but its area is necessarily bounded by the outermost ring. Unfortunately, added rings beyond the 3rd, 4th, or 5th, depending on the fill ratio, tend to have a dramatically reduced effect.